The most significant source of noise in a cellular system is interference. In the absence of out-of-cell and inter-cell interference, the theoretical capacity of wireless systems is extremely large, limited only by the thermal noise. As such, there is a need for technology capable of rejecting out-of-cell interference.
For any receiver, interference can be either caused by out-of-cell transmitters operating on the same frequency, or by transmitter(s) operating in the same cell, such as that found in code division multiple access (CDMA). On some occasions, echoes of the signal destined to a user can cause interference, which is often referred to as inter-symbol interference (ISI). This scenario will be referred to in general as self-interference. In other cases, signals destined to other receivers are the cause of interference.
A standard approach to combat interference is to share time, frequency, and spatial resources amongst different users. An example is “frequency planning,” where neighboring cells do not operate on the same frequency at the same time. This approach is employed in current time division multiple access (TDMA) systems, but is far from being efficient as it suffers a significant bandwidth penalty.
An interesting approach to combat interference in cellular systems is multi-user detection (MUD) in combination with CDMA. This is difficult to implement at the receiver end, since transmissions from different base stations do not arrive synchronously at the receiver. Moreover, the mobile terminals are usually small and power limited, making it difficult to implement algorithms requiring the large amount of computations typically required in MUD techniques.
CDMA technology suffers from self-interference and the interference from the signals destined to other users within a single cell. This can be overcome to some extent if the transmission sequences for different users within a cell are orthogonal, as is the case in high data rate (HDR) systems. However, out-of-cell interference remains a major issue. In HDR systems, more than half of the users experience signal to interference plus noise ratios (SINR) of less than zero dB. This interference limits the data rates that can be provided to these users if the system treats the interference as noise.
An efficient cellular system should enjoy a frequency reuse factor equal to one, meaning that all the available bandwidth is used in every cell. Such an implementation reduces the signal-to-noise ratio (SNR) of the users closer to the edge, or border, of each cell. For these mobile terminals at or near the border of adjacent cells, the desired signal from a servicing base station has almost the same power as the signals transmitted from the neighboring base stations, since the distances of the mobile terminal from the base stations are almost equal. If the interference is treated as noise, this low SINR limits the data rate that can be provided to these remote units. Because of the fairness factor of the system, admission of these mobile terminals into the system will limit maximum through-put. Hence, even those mobile terminals enjoying strong SINR will be penalized. The result is a highly inefficient system.
It may seem that the aforementioned problem can be overcome by rejecting the users with low SINR. However, this is not a practical solution, as any commercial wireless data solution has to be able to provide services to users at any location in the cell. In this light, a solution must be provided that improves the data rate of the users at the edge of each cell even in the presence of strong interference.
Another major impairment to a wireless channel is fading. Fading is caused by the destructive addition of the reflections of the desired signal. When a receiver is in fade, it cannot receive the desired signal. Given these issues there is a need for an efficient communication system capable of combating interference and fading in an effective manner while maintaining high data rates.